1. Field of the Invention
This invention relates to a solid polymer electrolyte and a method for manufacturing thereof. Specifically, the invention is related to a low cost solid polymer electrolyte and a method of producing such a solid polymer electrolyte, having excellent ionic conductivity suitable for high energy density batteries, such as lithium batteries, which is easy to manufacture and having superior mechanical properties.
2. Technical Background of the Invention
In recent year, there has been an increasing need for small rechargeable batteries for portable electronic instruments, which would be portable and offer high energy density. A representative example is lithium secondary batteries. However, lithium batteries now available in the marketplace are based on organic liquid electrolytes, and there remain many problems, such as leaking of electrolyte liquid and dendrite shorts. There are therefore a strong need to develop solid electrolyte based batteries.
Solid electrolytes of two different types are known: one type is based on inorganic materials and the other type is based on polymeric materials. Because it is cost effective to produce large surface areas with polymeric materials, high capacity batteries having a large surface area can be more readily produced using a polymeric electrolyte material than using an inorganic electrolyte material which must be produced by some vacuum process such as sputtering. It can also be expected that polymer electrolyte materials would lead to lowering of the production cost. Such an expectation is further enhanced because of the special properties of polymers that they can be produced in a variety of forms including a flexible sheet form, thus permitting portable batteries of the required shapes to be produced readily, and enabling customization of portable power driven electronic instruments to be produced economically.
In solid polymer electrolyte materials, much effort has been expended in studying polyether group of materials such as polyethylene oxide. These solid electrolytes are the type which operates by the movement of ions surrounded by the polymer chains, activated by the thermal motion (segment motion) of the polymer chains (R. Spinder and D. F. Shriver, J. Amer, Chem. Soc., 21, 648 (1988)).
However, such solid polymer electrolyte materials are faced with the limitations on the segment movement capability of the polymer chains near room temperatures, and it was difficult to generate high ionic conductivity over 10.sup.-4 S/cm where S denotes an inverse resistivity. Further, to increase the ionic conductivity by increasing the molecular chain motion of the polymer, it is necessary to decrease the molecular weight or soften the polymer, resulting in a large reduction in the mechanical properties of the solid polymer electrolyte. Thus, in spite of examples like lithium batteries, it has not been possible to obtain high capacity batteries, because of the low ionic conductivity of the solid polymer electrolyte at room temperature.
Additionally in recent years, there have been reports of an electrolyte made by impregnating a metallic salt electrolyte solution into polar polymers (polyacrylonitrile) (M. Watanabe et al, J. Polym. Sci. Polym. Phys., 21, 939 (1983)), and an electrolyte made by cross linking a composite, containing polar polymers, electrolyte solution and a light sensitive cross-linking agent by irradiating it with ultraviolet rays (K. M. Abraham and M. Alamgir, J. Electrochem. Soc., 137, 1657 (1990)). Among these samples, there are some which exhibited ionic conductivities in excess of 10.sup.-3 S/cm, but the mechanical properties of the material suffered because the impregnation of electrolyte into polar polymers resulted in softening of the polymer. Also, irradiation of ultraviolet rays is a dangerous and high cost step of manufacturing which needed to be addressed.